Organic light emitting diodes (OLED) have been increasingly used as current-type light-emitting devices in high-performance Active Matrix Organic Light Emitting Diode displays. With increasing of display size, conventional passive matrix organic light emitting diode displays require a shorter driving time for a single pixel, and thus require an increased transient current, which causes increased power consumption. Meanwhile, a voltage drop on a line of nanometer indium tin oxide (ITO) will be too large when a large current is applied, such that an operating voltage of OLED is too high and efficiency of OLED is decreased. The currents for OLEDs are input to active-matrix organic light-emitting diode displays when switching transistors are scanned progressively, which can solve the above problems well.
In design of an AMOLED backboard, a main problem to be solved is non-uniformity of luminance among respective pixels of AMOLED.
First, for AMOLED, pixel circuits are constituted by thin film transistors to supply currents for driving OLED devices, respectively. In prior art, Low-temperature poly-silicon thin film transistors (LTPS TFT) or oxide thin film transistor (Oxide TFT) are mostly adopted. Compared to a general amorphous silicon thin film transistor (amorphous-Si TFT), LTPS TFT and Oxide TFT have higher mobility and more stable characteristics, and thus are more suitable for AMOLED display. However, due to limitations of the crystallization process, LTPS TFTs produced on a large-area glass substrate often have non-uniformity on electrical parameters such as threshold voltage, mobility and the like, and such non-uniformity may cause current difference and luminance difference among OLED devices, that is, a mura phenomena occurs, which may be perceived by human eyes. Although process of Oxide TFTs achieves a better uniformity, similar to a-Si TFTs, a threshold voltage of Oxide TFT may drift under a high temperature or supplied with a voltage for a long time. Due to different images as displayed, drifts of threshold voltages of TFTs in respective areas on a panel may be different from each other, which may cause display luminance difference, such display luminance difference often renders in turn an image sticking phenomenon since such display luminance difference has a relation to a previously displayed image.
Second, in large-size display applications, since a certain resistance exists in a power supply line on the backboard, and driving currents for all pixels are supplied from an ARVDD power supply, a supply voltage for an area near to a location of the ARVDD power supply is higher than a supply voltage for an area far from the location, such a phenomenon is known as a voltage drop of the power supply (IR Drop). As the voltage of the ARVDD power supply has a relation to currents in different areas, IR drop may also cause driving current differences in different areas, and thus a mura phenomenon appears during display. LTPS process for constructing pixel units by adopting P-type TFTs is sensitive to such an IP drop since a storage capacitor therein is connected between the ARVDD and a gate of TFT, and thus voltage variation of ARVDD may directly affect a gate-source voltage Vgs for driving the TFT.
Third, the non-uniformity of the electrical characteristics of the OLED devices may also be resulted from non-uniform thickness of the mask during an evaporation process. For the a-Si or Oxide TFT process constructing pixel units by adopting N-type TFTs, a storage capacitor therein is connected between a gate of a driving TFT and an anode of the light-emitting device, if voltages at the anodes of the OLED devices of respective pixels are different when a data voltage is transmitted to the gates, the gate-source voltages Vgs actually applied to the TFTs may be different, so that display luminance are different due to different driving currents.
In prior art, an AMOLED voltage-type pixel unit driving circuit is provided, and a voltage-type driving method is similar to a conventional AMLCD driving method, wherein a voltage signal representative of a gray scale is supplied from a driving unit, and the voltage signal is converted to a current signal for a driving transistor inside the pixel circuit so as to drive OLED to achieve the luminance corresponding to the gray scale. Such a method has advantages of fast driving speed and simple implementation, is suitable to be used in the driving of a large size panel, and is thus widely adopted in display industry. However, it is necessary to design additional devices comprising TFTs and capacitors to compensate for non-uniformity of TFTs, IR Drop and non-uniformity of OLEDs.
As shown in FIG. 1, a conventional circuit configuration of a voltage-driven type pixel unit adopts two TFTs and a capacitor (2T1C), wherein a data voltage on a data line is transmitted to a gate of a driving transistor TQ through a switching transistor TK and is then converted into a corresponding current by the driving transistor TQ for being supplied to an OLED device. In normal operation, the driving transistor TQ is in a saturation region and provides a constant current during a scanning period for a row of pixels. The current may be represented as:
      I          O      ⁢                          ⁢      L      ⁢                          ⁢      E      ⁢                          ⁢      D        =            1      2        ⁢                  μ        n            ·      Cox      ·              W        L            ·                        (                                    V              ⁢                                                          ⁢              data                        -                          V              ⁢                                                          ⁢              oled                        -                          V              ⁢                                                          ⁢              thn                                )                2            
Wherein, for all pixel units, μn represents a carrier mobility, Cox represents a gate oxide layer capacitance, W/L represents a width/length ratio of a channel of a transistor, Vdata represents a data voltage, Voled represents an operating voltage of OLED, Vthn represents a threshold voltage of the transistor, wherein Vthn has a positive value when the transistor is an enhanced TFT and has a negative value when the transistor is a depleted TFT.
Although the pixel unit driving circuit in the prior art has been widely used, it has the following problems inevitably: there is difference among currents in different pixel units if the different pixel units have different Vthn; in addition, if Vthn in a pixel unit drifts with elapse of time, the current may vary with time, thus rendering an image sticking; moreover, the non-uniformity of the OLED devices will render different operating voltages of the OLED devices, which may also contribute to the difference among currents of different pixel units.